Kentucky Derby winner Animal Kingdom, who came up lame in the days following the Belmont Stakes, has a slab fracture in his left hind leg that will require surgery. He will not race again this season, according to Team Valor CEO Barry Irwin and trainer Graham Motion.

Animal Kingdom, who resides at the Fair Hill Training Facility in Elkton, had x-rays performed on his leg two weeks ago at the New Bolton Research Center in Kennett Square, Penn., and veterinarians detected a small defect in his leg — a fissure — they wanted to closely monitor. Another round of x-rays this week revealed a fracture just above the fissure on his left hock, an injury that will require surgery but hopefully not end his racing career.

Animal Kingdom will have surgery on Thursday morning. Dr. Dean Richardson of New Bolton will insert a screw into the bone that will hopefully compress the fracture to aid in the healing process. After coming out of surgery, the Derby winner will rest for four to six weeks and then begin rehabilitation on a water treadmill.

"To be honest, it's something we could leave alone and not do surgery one," Motion said. "But when you do surgery on it, the healing happens much quicker and is more definitive. It takes some strain off the joints. We also couldn't rehab him on the water treadmill if we elected not to do the surgery."

A slab fracture is not an unheard of injury for a thoroughbred, and horses can return to top form with rest and rehab. Lorne Dancer, a highly-regarded horse in Australia, recently returned from a similar injury to win several big races.

Animal Kingdom is the latest in a string of 3-year-olds who have been sidelined this year with injury, joining Uncle Mo, Nehro, Dialed In, Premier Pegasus, To Honor and Serve, Archarcharch and Toby's Corner.

Motion, in particular, has experienced more than his share if bad luck. Toby's Corner looked like a serious Derby contender the week before the Run for the Roses, but the day before he was scheduled to ship to Louisville, he came up lame. The following week he looked fine. Veterinarians still haven't been able to determine what was wrong with him, but he still hasn't returned to training.

"Ironically, it was the same left hind leg that Animal Kingdom has," Motion said. "It's definitely disappointing to have that happen to two of the top 3-year-olds, just because you want them to show their potential. You're not going to replace two horses like that. It's a little bittersweet with Animal Kingdom, because he didn't even have the chance to run his race in the Belmont. But I feel extremely fortunate to have experienced the success we've had this year."

Motion said he's hopeful that Animal Kingdom will return to racing at the Dubai World Cup in March of 2012.

"Ultimately that's what we've been looking forward to running him in since we realized how good he was," Motion said.

Irwin said it's possible Animal Kingdom could run in a prep race prior to the World Cup, but he didn't believe it was necessary. So it may be awhile before the public sees the Derby winner run again.

"Graham and I have talked about this," Irwin said. "No decision will be made for awhile. But running Animal Kingdom in the World Cup without a prep is definitely something to consider."

A record Preakness crowd saw the Kentucky Derby winner streak to victory Saturday at Pimlico Race Course, tantalizing fans with the possibility that California Chrome will be the first horse in nearly four decades to win racing's coveted Triple Crown.

InfieldFest concerts Infield party scene Infield debauchery Celebrities Fashion Bikini contest Also see: VIDEOS | Your #PREAKNESS PHOTOS | ALL GALLERIES A look at the horses the field for the 139th Preakness, to be run Saturday at Pimlico Race Course. NOTE: Field ordered by morning-line odds,...